Articles packaged in a bundle, on a pallet or in a crate for shipping, storage and merchandising are often secured with a steel or polymer strap applied in a tensioned loop by an automatic or manually operated strapping machine. Some applications, and in particular those applications in which the strap secures a package having substantial weight such as a stack of bricks, require the use of a steel strap which has greater tensile strength and is less susceptible to deterioration by abrasion than polymer and existing non-heat treated metal strap. Further, although certain existing steel strap is readily applicable to heavy packaged articles having cylindrical shapes and otherwise smooth or obtuse surfaces, there are limitations on the extent to which it can be formed under tension over and around sharp edges and corners of a package. More specifically, packages having sharp edges or corners with a small radius of curvature, for example a 90 degree corner, pose a problem for existing steel strap because the strap is subject to tremendous stress and strain as the strap tension is increased to an extent necessary to secure the packaged article. This stress and strain frequently causes the strap to fracture proximate to the edge or corner of the packaged article. Particularly, the relatively low ductility of non-heat treated strap contributes to the failure of strap used in this application. This problem may be particularly severe in applications in which the strap is applied and tensioned with an automatic strapping machine which generates a high tension in a short time interval during a rapid strap application process. Steel strap failure may be prevented by reducing the tension applied to the strap, but reducing strap tension may result in insecurely packaged articles and is therefore often an unacceptable alternative. It has also been suggested to place a shield between sharp edges or corners of the packaged articles and the strap to reduce strap stress and strain, but this requires an additional step that is time consuming and not readily integratable in an automated strapping operation. Further, in many applications, the placement of a shield between the package corners and the strap may not positively offset strap forces in a manner to prevent strap fracture or failure.
The physical properties of the steel that comprises the steel strap determine the tensile strength and the formability of the strap. Iron based material suitable for steel strap generally includes carbon which is added to the steel to increase the tensile strength of the strap. The addition of carbon however also tends to increase steel embrittlement which decreases steel formability and, accordingly, the ability of steel strap to be formed over and around corners without fracturing. Free nitrogen is another source of steel embrittlement as well as a source of strain aging, which causes steel to kink during plastic deformation. In low carbon steels, the detrimental effects of free nitrogen overshadow the detrimental effects of carbon. These detrimental effects generally result from the ability of free nitrogen and carbon to diffuse rapidly throughout the structure of the steel and congregate in imperfections or dislocations that occur in the lattice structure when the steel is subject to plastic deformation. The congregation of free nitrogen and, to a lesser extent, carbon atoms prevents the individual atoms of the lattice structure from moving about the lattice structure from one position to the next position, and a result of this congregation is that the lattice structure will lock up and fracture rather than undergo plastic deformation.
It is known that adding elements like titanium, zirconium, or boron to steel will scavenge free nitrogen, that is, remove detrimental amounts of free nitrogen from the steel by reacting with the free nitrogen to form titanium nitride, zirconium nitride, or boron nitride, respectively. Removal of free nitrogen results in improved formability and ductility, decreased work hardening and the elimination of nitrogen related strain aging. The formation of nitrides of titanium and zirconium, however, are known to cause fracture of the steel matrix during cold reduction, and to decrease residual ductility in full hard cold rolled steels. Further, the addition of nitrogen scavenging elements in amounts in excess of that required for free nitrogen scavenging will increase the hardenability and decrease the formability of the steel, and further additions may result in embrittlement.
It is also known that reduction of steel by cold working will increase steel tensile strength thereby allowing a decrease in the carbon content while still maintaining a fixed tensile strength. But reduction of steel by cold working also increases steel embrittlement and decreases steel formability. In applications where steel formability is important, therefore, reduction by cold working has been performed to a limited extent to avoid embrittlement and the attending loss in steel formability. For example, in pipe manufacturing processes, reduction of steel by cold working is limited to approximately 10 percent because further reduction by cold working would increase steel embrittlement to the extent that the steel could not be shaped into a pipe without fracturing.
It is well known in the art to improve the tensile strength of steel by alloying with other elements, or by heat treatment. However, those methods are relatively costly, and are not generally appropriate solutions to the problems associated with steel strapping. Also, the heat treating requires energy input, thereby exacerbating the ever present energy shortage.